Abstract

An investigation of 125 storms (67 thunderstorms and 58 shower clouds with high electric fields) completed during 1945–1948 at the Zugspitze Observatory in Germany (10,000 ft) is presented. The results are discussed with regard to electrical conditions, precipitation, and some other meteorological details. Approximately 80 per cent of the observations were made inside clouds. The more important aspects of the results concern the following points:

1. Average position of space charges inside thunderclouds. A local positive charge is found precisely at the freezing level; it is located in the center of precipitation and lightning and has a restricted horizontal depth (order of magnitude: 1 km). In the same vertical air column, but at lower temperatures (around −8C), the center of the negative main charge is evidenced. The center of the positive main charge appears to be outside this air column, following at a distance somewhat behind the negative charge and at a higher level.

2. Role of precipitation in thunderclouds. Solid precipitation prevails for the greatest part inside the clouds. In the precipitation center, graupel (especially in the form of snow pellets), consisting of frozen cloud droplets, is the basic type of hydrometeor. Hail is relatively rare. During the later periods of a storm, snow crystals (formed from water vapor by sublimation) prevail in the area of more steady and moderate precipitation. Ice particles inside the clouds are usually oppositely charged to the simultaneous potential gradient.

3. Relation between space charges and precipitation. There is some evidence that in mature thunderstorm cells two vertical dipoles follow each other: (a) a “graupel dipole” with a more concentrated upper charge of negative sign and (b) a more elevated “snow dipole” with a concentrated upper positive charge. The lightning exchange between these upper space-charges gives the illusion of one slanted dipole.

The distribution of charges indicates that the two upper space-charges are attached to the cloud droplets, the weaker lower charges to the precipitation. This indicates a preferred positive sign of the graupel particles and a preferred negative sign of snow crystals.

The possibility is considered that the graupel dipole is formed by a primary process and gives rise to an electric field in which the snow dipole develops by a secondary process.

4. Cell structure and parcel structure. Meteorological and electrical records indicate that the individual cells in a thunderstorm are further sub-divided into units which are approximately one order of magnitude smaller in linear dimension. These parcels may be related to turbulence systems and may account for the local character of high electric fields and for the complicated structure of thunderstorms.

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